The present invention relates to a process for producing a partially pressure-sensitive recording paper, and more in detail, the present invention relates to a process for producing a partially pressure-sensitive recording paper prepared by painting an ink comprising thermally melting suspention medium and microcapsules encapsulating a colourless solution of a colour-former on a specified part of a surface of a sheet of paper.
In recent years, the recording papers for computer or businessform such as slips for business have come to be more and more complicated and diversified in accordance with the improvement of business efficiency and mechanization of business, and a considerable number of the businessforms take the form including a plurality of copying sheets.
Although in such cases, the pressure sensitive recording paper is frequently used, the conventional pressure sensitive recording paper is made by superposing an upper sheet of paper (hereinafter referred to as CB sheet) having the back surface thereof painted with microcapsules containing a solution of the so-called leucotype dyestuff (as a colour-former) as the core substance onto a lower sheet of paper (hereinafter referred to as CF sheet) having the front surface thereof painted with acid clay or an acidic resin (as a developer), and in the actual use, by the application of writing pressure by a pen or printing pressure by a typewriter, the microcapsules at the pressed part(s) are broken to contact the colour former with the developer, thereby causing coloration and resulting in printing of the handwritten or typewritten items. In the case of pressure sensitive recording paper having a number of copying sheets, one or more sheets of paper each having both the front surface thereof painted with a developer and the back surface thereof painted with microcapsules containing a colour former (hereinafter referred to as CBF sheet(s)) is or are inserted between the CB sheet and the CF sheet, and the thus prepared multi-layered paper is used for the purpose.
However, since the microcapsules have been painted on the whole surface of the sheet material, it is necessary to desensitize the developer on the part(s) of the surface of the sheet material by the use of a reducer ink (de-sensitizing ink) or the like in the cases where only a specified number of the sheets of recording paper should be copied or only a specified part(s) of a slip should be copied, for preventing the unnecessary copying. Such a technique would require much labor and it is inevitable to waste the microcapsules on the part(s) not to be copied or on the number of sheets of copying paper by the de-sensitizing.
In view of the above-mentioned demerit of the conventional pressure sensitive recording paper, it is considered that a recording paper which is partially pressure-sensitized can be obtained without using any de-sensitizing ink, if it is possible to retain the microcapsules only on the really necessary part of the surface of the CB sheet. Namely, in the case where a pressure sensitive recording paper having the microcapsules painted only on the necessary part(s) of the surface thereof can be prepared by a spot-printing method or the like, a really epoch-making pressure sensitive recording paper can be offered.
However, since at present the conventional pressure sensitive recording paper is prepared by painting a surface of base paper with an aqueous slurry-like material comprising a water-soluble binder, water as a solvent, adjuvants and the microcapsules, it is practically almost impossible to apply such a technique mentioned above to the spot-printing and the like, because the thus spot-printed sheet of paper partly wrinkles on drying.
On the other hand, from the economical viewpoint of not using the expensive microcapsules and the technical merits of copy-printing only on the necessary part of the surface, the development of the recording paper which is partially pressure-sensitized has been eagerly requested.
In answering the request, the processes for preparing the recording paper which is partially pressure-sensitized have been developed. As an example, a process wherein the microcapsules are dispersed in an organic solvent containing a vehicle and the thus obtained dispersion is painted only on the specified part(s) of the surface of sheet material by a printing machine of photogravure type or flexo type has been known.
However, such a printing machine cannot be installed easily in any optional place. On the recording paper which is partially pressure-sensitized obtained by the process of painting a dispersion of microcapsules in a solution of a vehicle on a sheet of paper and drying the thus painted paper, the whole surface of the microcapsule is covered by a binder and the microcapsules are fixed onto the surface of the sheet of paper and accordingly, the breaking efficiency of the microcapsules and the transfer efficiency from CB sheet to CF sheet are inhibited resulting in the necessity of painting a large amount of the microcapsules for obtaining a predetermined color density.
Although there is another method for preparing the partially pressure-sensitive recording paper, in which after partially painting a photo-setting adhesive substance on a specified part of a base sheet of paper, the microcapsules are fixed to the thus painted part before the adhesive substance solidifies and then the adhesive substance is made to solidify, since the adhesive substance is highly expensive and it is necessary to install an irradiating apparatus for photo-setting the adhesive substance, there are demerits of the relatively high cost of the thus obtained partially pressure-sensitive recording paper and the troublesome operation of preparing thereof.
In addition, as the partially pressure-sensitive recording paper prepared by using a conventional thermally melting ink, those prepared by partially painting a so-called carbon ink made by suspending a coloured pigment such as carbon black, etc. in the thermally melting ink on a base sheet of paper have been well known and broadly used because of the simplicity of printing and the low price of the product. However, the problem of soiling the cloth and the hands of the user thereof could not have been solved because of the ink obtained by only mixing the coloured pigment with the wax. Several methods of reducing such soiling as far as possible have been devised as follows.
For instance, a light-coloured pigment of light cobalt or light blue in colour is used, or on the other hand, a dark black pigment is used in preparing a hard carbon ink for use under a relatively strong pressure in copying. However, any methods devised hitherto were not sufficient to prevent the soiling of the cloth and the hands.
On the other hand, concerning the pressure-sensitive recording paper prepared by using a thermally melting ink containing the microcapsules encapsulating a colourless solution of a colour-former, those entitled with the pressure-sensitive recording paper of a non-carbon type have been disclosed in U.S. Pat. No. 3,016,308, Japanese Patent Application Laying Open No. 53-11610 and Japanese Patent Application Laying Open No. 53-135720.
For instance, U.S. Pat. No. 3,016,308 discloses a pressure-sensitive recording paper prepared by painting a thermally melting ink containing microcapsules obtained by spray-drying a dispersion of a solution of a colour-former in a solution in which a material for wall membrane of the microcapsule has been dissolved. However, in the case of the microcapsules obtained by the spray-drying method since the wall membrane thereof within at most a few minutes is formed, the wall membrane is poor in the compactness and accordingly, it is difficult to retain the material encapsulated therein safely during the preparation of the ink or the preservation thereof.
The method disclosed in Japanese Patent Applications Laying Open Nos. 53-11610 and 53-135720 comprises the steps of mixing an aqueous slurry of the microcapsules with a fluid thermally melting suspension medium, removing the moisture or the volatile organic solvent therefrom under a reduced pressure, thereby obtaining a non-aqueous and thermally melting ink in which the microcapsules have been dispersed, and painting the thus obtained ink on a sheet of paper. However, since it is necessary to carry out heating under a reduced pressure for the removal of the moisture from the ink by evaporation together with the vigorous stirring, thus such a treatment inevitally causing the breakdown of the microcapsules, there is a demerit of low in the colour-developing efficiency.
Namely, one of the reasons why a partially pressure-sensitive recording paper utilizing a thermally melting ink and showing a high colour-developing efficiency could not have been obtained is based on the fact that in the case of preparing the microcapsules, the wall membrane thereof is subjected to severe conditions due to the necessity of removing the aqueous medium because of the necessity of uniformly dispersing the thus prepared microcapsules in a thermally melting suspension medium for making the ink.
In consideration of the above-mentioned problems, the present inventors have studied the method for producing the partially pressure-sensitive recording paper of a high colour-developing efficiency at a low price, and as a result, they have succeeded in obtaining the partially pressure-sensitive recording paper of a high colour-developing efficiency by easily painting a thermally melting ink containing the microcapsules having a wall membrane comprising an aminoplast, which are easily separable from the aqueous medium used in preparing the microcapsules, while using a conventional printing machine for painting a conventional thermally melting ink.